Camshafts are used in internal combustion engines in order to actuate gas exchange valves. The camshaft in an internal combustion engine includes a plurality of cams that engage cam followers (i.e. bucket tappets, finger levers or rocker arms). When the camshaft rotates, the cams lift or depress the cam followers which in turn actuate gas exchange valves (intake, exhaust). The position and shape of the cams dictate the opening period and amplitude as well as the opening and closing time of the gas exchange valves.
Separate intake and exhaust camshaft assemblies are known in which each camshaft and its related cam lobes separately operate intake valves and exhaust valves, respectively.
Concentric camshaft assemblies are also known in which separate intake and exhaust camshafts are concentrically arranged by providing a hollow outer camshaft in which an inner camshaft is located, with the inner camshaft cam lobes being rotatable on the outer camshaft, and connected through slots in the hollow outer camshaft to the inner camshaft. This allows the use of separate camshafts for intake and exhaust valve actuation within generally the same space required for a single camshaft.
Camshaft phasers are used to advance or retard the opening or closing period, phasing the camshaft with respect to the crankshaft rotation. Camshaft phasers generally comprise a timing gear, which can be a chain, belt or gear wheel connected in fixed rotation to a crankshaft by a chain, belt or gear drive, respectively, acting as an input to the phaser. The phaser includes an output connection to the inner or outer camshaft in a concentric camshaft arrangement, or, alternatively, an output connection to an exhaust or intake camshaft. A phasing input is also provided in the form of a hydraulic, pneumatic or electric drive in order to phase or adjust the output rotation of the camshaft relative to the input rotation of the crankshaft.
Camshaft phasers are generally known in two forms, a piston-type phaser with an axially displaceable piston and a vane-type phaser with vanes that can be acted upon and pivoted in the circumferential direction. With either type, the camshaft phaser is fixedly mounted on the end of a camshaft. Camshaft phasers that operate according to the vane-cell principle for use on single camshafts are known in the art. It is also known to use camshaft phasers in connection with concentric camshaft assemblies for controlling the phase position of the inner camshaft, the outer camshaft, or both relative to each other.
Vane-cell type phasers employ a supply of hydraulic fluid, normally engine oil, to opposing chambers in the phaser in order to shift the vanes within the phaser circumferentially and thus selectively phase cam timing. Camshaft phasers are subject to oil loss from the phaser through leakage. During normal engine operation engine oil pressure generated by the engine oil pump is sufficient to keep the cam phaser full of oil and, therefore, functioning properly. However, when the engine is not operating, oil leakage from the cam phaser may leave the cam phaser chambers filled with air. This lack of controlling oil pressure and the presence of air in the chambers during engine start conditions, before the engine oil pump generates enough oil pressure and flow, may cause the phaser to oscillate excessively due to lack of oil. This oscillation may, in turn cause noise or damage to the cam phaser mechanism. In addition, it is desirable to have the cam phaser locked in a particular position during engine start-up.